MS/MS spectra were recorded for the ions of [Glu1]-fibrinopeptide produced upon radiolysis in O16 and O18-labeled water that correspond to unmodified peptide, and peptide modified with O16 and O18. These spectra reveal that phenylalanine residues are modified with both O16 and O18 as indicated by the shift in m/z values for the y-type ions y4 through y11 and N-terminal b-type ions b10 through b13. Both phenylalanine residues are partially modified as indicated by the presence of the y4 and b10 fragments that do not contain O16/ O18 in spectra for modified peptide. Mass spectrometry studies of the radiolysis products of other peptides in O18-labeled water reveal that methionine reacts with hydroxyl radicals and oxygen while phenylalanine and tyrosine residues react predominantly with hydroxyl radicals. Proline is oxidized solely by molecular oxygen to produce +14 and +16u products presumably through radical ion formation by hydrated electrons, followed by reactions with O2. The loss of molecular hydrogen to a ketone accounts for the formation of the +14 radiolyzed product. Tryptophan shows multiple oxidation to form products at +16 and +32u. Cysteine is shown to incorporate up to three oxygens. Radiolysis in O18-labeled water revealed that hydroxyl radicals and molecular oxygen both compete in the formation of these products.